STUDY OF NANOSECOND ND: YAG 213 nm LASER ABLATION OF EXTRA HEAVY CRUDE OIL BY INDUCTIVELY COUPLED...

8/14/2019 STUDY OF NANOSECOND ND: YAG 213 nm LASER ABLATION OF EXTRA HEAVY CRUDE OIL BY INDUCTIVELY COUPLE

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Henry Wuyke -et al., IJSIT, 2013, 2(5),367-381

IJSIT (www.ijsit.com), Volume 2, Issue 5, September-October 2013 367

STUDY OF NANOSECOND ND: YAG 213 nm LASER ABLATION OF EXTRA

HEAVY CRUDE OIL BY INDUCTIVELY COUPLED PLASMA MASS

SPECTROMETRY

Henry Wuyke*, LlinaberFeo, Miguel Murillo

*PDVSA-INTEVEP S.A. Urbanizacion Santa Rosa. Sector El Tambor. Los Teques. Estado Miranda. Apdo.

76343, Caracas 1070A, Venezuela

Centro de QumicaAnaltica, Facultad de Ciencias, Universidad Central de Venezuela, Apdo.47102,

Caracas 1070A, Venezuela.

ABSTRACT

The Laser-Extra Heavy Crude Oil interaction was studied using a nanosecond Nd:YAG 213 nm laser

coupled to an ICP-MS. For this study, ~2.7 g Extra Heavy Crude Oil was mixed with ~3 mL xylene and 0,3 g

organic multielemental solution containing 100 g g-1of Ag, Al, Ba, Cd, Cr, Cu, Mg, Mn, Mo, Ni, Pb, Sn, Ti, V, and

Zn, xylene was left to evaporate at 90 C and multielemental samples were obtained and analyzed by LA-ICP-

MS. Laser parameters, such as, ablation gas flow, sample scanning speed, spot size, fluence and focus position

were optimized and RSD (n = 8) from 3% to 5% for the net signal of 63Cu, 95Mo, 60Ni and 66Zn and from 0.9%

to 3.3% for the ratios Ni/Mo and Cu/Zn were obtained. Elemental fractionation was observed with both Zn

and Cd when fluence was varied. Nevertheless, evidence is not enough to explain results as a consequence of

high volatilityor highionization potential (PI)of these elements. Ablationcratersobtained showsigns ofmore

pronounced thermal processesat higher fluences. Elemental fractionation was also observed in the ICP due to

mass load effect. Isotopic ratios 95Mo/112Cd, 63Cu/66Zn y 27Al/24Mg, which have different ionizationpotential

(1.89; 1.68 and 1.66 eV, respectively expressed as PIdenominator- PInumerator) increase when mass load is increase.On the other hand, 60Ni/63Cu ratio (0.09 eV difference) remains constant. These results indicate that the

plasma ionization temperature decreases when mass load is increased affecting

thoseelementswithhighionization potential.

Keywords:Laser Ablation, ICP-MS, Extra Heavy Crude Oil, Elemental Fractionation.


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INTRODUCTION

The use of LA-ICP-MS to analyze crude oil maybe advantageoustoeliminatethe extensivesample

treatmentstepsoftraditional methods such as calcination or acid digestion. However, few works using LA-ICP-

MS for direct elemental analysis in crude oil have been reported [1-3]. It can be probably due to difficulties

found during ablation, such as splashing, of this type of liquid samples. LA-ICP-MS is more suitable forsolid

samples [4-7]. Therefore, it is a potential tool for direct analysis of semi-solid extra heavy crude oil with less

trouble inthe ablation process.

The ablation processis strongly dependent onthe propertiesofthe sample such as composition,

thermal diffusivity and absorptivity [4].Therefore, matrix matched calibration standards or reference

certificated materials are required to ensure accuracyof results. However, calibration standards are not

available for the majority of sample matrixes, which limitsthe applicationsofLA-ICP-MS [8,9]. Therefore, it is

important to know how the laser ablation of extra heavy crude oil is affected by eachexperimental parameter

and identify which of them lead rise to fractionation or non-stoichiometric sampling.

Pulse length isoneofthe most important parametersinvolvedinmechanisms by whichthe ablation

laser processoccurs. Lasers with pulse length in nanosecond (ns) and femtosecond (fs) are the

mostfrequentlyused in LA-ICP-MS applications. Generally, fs lasers produce better results because the time

interval of energy delivering is much smaller than the time interval needed for the thermal stabilization of the

sample (intheorder of picoseconds) [6,10]. Furthermore, photon intensitiesreach valuesof >1014 W cm-2,

thereby processesare predominantlynon-thermal, causingless collateral damage, less dependence onthe

matrix, smaller particlesand nonlaser-plasma interaction [6]. Nevertheless, even thoughthe latest

developmentsare aimed to take advantageoffslasers, todaynslasers are still very populardue primarily totheir

low cost.

Fluence also plays an important role in ablation mechanisms. It must be above a threshold value to

lead the ablation process [10]. When ns lasers are used, fluence should not be too high to avoid the generation

of a high electron density plasma, because it increases the laser-plasma interaction and therefore, reduces the

interaction between the laser and the sample [11]. When fs lasers are used, fractionation appears to be only

near the threshold value of the sample and reach an asymptotic value as the fluence increases [9,12-14].

Fractionationaccording to the fluencein the rangeof2 to 42J cm-2

with afslaserin brass, aluminumandglass samples was assessed by Bian et al. [12].In those conditions, the ratio Zn/Cu in the brass and aluminum

samples was independent of the fluence while for glass samples there was a strong dependency especially for

the most transparent glass sample (NIST 610). However, at higher fluence, the ratio Zn/Cu reached an

expected asymptotic value, considering the results of brass and aluminum measures. Similar results on the

same glass sample type were obtained by Koch et al. [13]. Fractionation was observed with ratios Zn/Cu and


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Mo/Ni which have different ionization potentials but not with the ratio U/Th which have similar ionization

potentials (PI: U: 6.2 eV, Th: 6.3 eV y Pb: 7.5 eV), which evidence of that the ionization potential affects the

ablation process. It has been mentioned that Coulomb interactions are responsible for this behavior [11].

However, ablationmechanismsarestillnotfully understoodto the date [15]. There are other factors such as the

volatility of the elements that influence the signals when thermal processes occurred, i.e.morevolatileelements arevaporizedpreferentially,resulting in greatersignal and causingfractionation [12,16].

Fractionationhasalso beenstudiedas a function oftime, i.e. shot to shot. Garcia et al. [9] studied this

effect using afs laser in binarymetallic, semiconductorsand glasses samples. They obtainedthatthe

Cu/Znchangesfrom shot to shotreachedan asymptotic valueindependentof fluence,which correspondsto

astoichiometricsampling. Furthermore, thisasymptotic valueis reachedfaster ifhigh fluenceisapplied.

There are other parameters that also affect the ablation process such as ablation gas and laser

wavelength. Helium and argon are the most used gases for ablation environmental and sample

transportation. However, as argon has a lower ionization potential than helium, there is a greater likelihood

of plasma formation and shielding effect that prevent an efficient interaction between laser and sample, so

that helium is preferred [11]. Wlle et al. [17]using ns and fs lasers in glass, zircon and silicon samples,

observed a depression of the detection efficiency by a factor of 5 when used argon instead of helium as

ablation gas. Studies have shown that using shorter wavelengths, with both ns and fs lasers in glass samples

fractionation effects are reduced [13,18,19].

Particles generated in the ablation process can have different compositions in terms of size,

especially if a ns laser is used [6]. It creates fractionation if there is no transport efficiency, because large

particles (> 1 m) tend to be lost by gravity or deposited on chamber ablation. While, small particles (


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MATERIALS AND METHODS

Instrumentation:

Laser ablation was performed using a Nd:YAG UP-213 LA New Wave Research operated at 213 nm

with a pulsed duration of ~ 4 ns. An ICP-MS Agilent Technologies 7500ce that consist in a plasma source,

aoctopole reaction cell and a quadrupole mass analyzer was used. Laser ablation and ICP-MS were connected

using Tygon tubes (S-50-HL Class VI, ~ 3 mm i.d.) as shown in Fig. 1. In analysis, the aerosol from the ablation

cell is mixed with Ar using a "Y" connection, then goes to a cyclone chamber of ~ 4 cm diameter and finally to

the ICP-MS. The bypass and purge valves are used for analysis preconditioning.

Figure 1:Coupling laser ablation to ICP-MS

Reagents, stardards and samples:

Extra heavy crude oil samples (API < 10) were provided by PDVSA-INTEVEP. Xylene (puriss. p.a.)

Sigma-Aldrich and an organic multielemental solution containing 100 g g -1of Ag, Al, Ba, Cd, Cr, Cu, Mg, Mn,

Mo, Ni, Pb, Sn, Ti, V and Zn, S-21 Conostan, USA were used. The tuning solution to perform the ICP-MS was

preparedfrom ARMTM Absolute Standards, INC.1000 mg/Kg of Li, Mg, Co, Y, Ce and Pb in 2% HNO 3.

Sample preparation:

Approximately 2.7 g of oil was dissolved in a beaker with 2 mL xylene and 0.3 g of organic

multielemental solution, the mix was homogenized by magnetic stirring and brought to an oven at 90 C for 2

hours until evaporation of xylene. Finally, part of the oil was placed in a sample holder tray of 1 cm diameter

and 3 mm high and led to LA-ICP-MS. The resulting mixture contained a concentration of 10 g g -1in metals

from the organic multielemental solution.

General conditions of analysis:

In this work, several experiments were performed in order to study the laser - extra heavy crude oil

interaction by ICP-MS. Table 1 shows the general conditions of analysis. Isotopesof each elementwere chosen


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tominimizeisobaricandpolyatomicinterferences.

The ICP-MS parameters were those that resulted in good sensitivity for all elements of the tuning

solution, covering a wide tuning range of masses: 7Li, 24Mg, 59Co, 89Y, 140Ce and 208Pb. Once sample is

introduced into the ablation cell, the system is purged with He 1 L min -1flow for 5 min.

ICP-MS Agilent 7500ce

Plasma gas flow, L min-1 15

Foward power, W 1500

Sample depth, mm 6.5

Integration timeper point, s 0.1

Integration timeper mass, s 0.3 (3 point per mass)

Laser ablation New Wave Research UP-213 LA

Ablation gas flow (He), L min-1 0.75

Mixing gas flow (Ar), L min-1 0.5

Fluence, J cm-2 ~ 34 (100 %)

Focus, mm 4 (above the sample)

Spot size, m 100 (1000 m to 4 mm of defocus)

Pulse frequency, Hz 20

Scanning speed, m s-1(linear) 20

Table 1:General conditions of analysis

RESULTS AND DISCUSSION

A typical ablation profile of Ni, Cu, Zn and Mo is shown in Fig. 2. It is observed that the signal of Ni is

much greater than the signals of Cu, Zn and Mo. Thisis becauseNi isoneofthe major elementsinthecrude oil.

However, there is good signal stability for the four elements at different concentration levels. Therefore, Cu

and / or Zn signals were used to shown the optimization of the laser parameters and were selected for their

differences in ionization potential (PI) and melting point (mp) (PI/mp: Cu 7.72 eV / 1083 C, Zn 9.42 eV / 419

C).


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Figure 2:Ablation profile of 60Ni, 63Cu, 66Zn and 95Mo at general analysis conditions. Ablation time was 50 s

with 10 s of delay relative to the acquisition start

Optimization of the laser parameters was performed with the aim of achieving the greatest intensity

for 60Ni, 63Cu, 66Zn and 95Mo isotopes while maintaining good repeatability. In experiments with He-Ar as

ablation gas and mixing gas respectively, flow rates were optimized between 0.4 0.9 L min-1and the best

results were obtained with 0.75 L min-1 He and 0.5 L min-1 Ar. In the Ar-Ar system, flow rates were optimized

between 0 to 1.2 L / min, and the best results were obtained with 1.0 L min -1Ar (ablation gas) and 0.2 L min-

1Ar (mixing gas).

Both He and Ar as ablation gases were compared in terms of the transient signal produced. The

results are shown in Fig. 3. Figures 3a and 3c show that 63Cu and 66Zn signals are greater when He is used as

ablation gas. Furthermore, a more pronounced memory effect is observed when Ar is used as ablation gas

(figures 3b and 3d).

The damage on crude oil surface is greater when Ar is used in the ablation cell (figure 4) at both

optimum analysis conditions (4 mm of defocus) and focus. These results are consistent with previous studies

[11,17], where it is argued that Ar, which posses an ionization potential (15.7 eV) lower than He (24.6 eV),

promotes the formation of plasma on the surface, which interacts with the sample and deform it, i.e. there

aresignificantthermal processes.

The memory effect produced when Ar is used can be explained by the expansion to large volume of

the ablated material into the ablation cell. It makes that the system takes longer to transport this volume. It

has been mentioned that this expansion is more pronounced when the ionization potential is decreased [11].

It agrees with the results obtained in this study.


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He was chosen as ablation gas for the subsequent experiments because it produces a significantly

high signal for Cu, Zn, Ni and Mo (Student t test with = 0.05) and an improved precision to the Cu/Zn and

Ni/Mo ratios (Fisher F test = 0.05) when compared to the obtained with Ar as ablation gas. Results are

shown in table 2.

Figure 3:Signal versus time for two ablation gases, He and Ar: a and b) CPS of 63Cu, c and d) CPS of 66Zn. a and

c) approximately at 10 s, the laser was switched on 7s (140 pulses), b and d) zoom between times 30 and 90 s


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Figure 4:Picturesof cratersat different ablation conditions. It is indicated in order: ablation gas, pulses

number and defocus level (mm): a) He, 1000 pulses, 4 mm, b) Ar, 1000 pulses, 4 mm, c) He, 400 pulses, 0 mm

(in focus), d) Ar, 100 pulses, 0 mm (in focus). The mark corresponds to 100 m. He and Ar flow in the ablation

cell were 0.75 L min-1

Element o

ratio

Signal

Average

(CPS). He

SD (CPS).

HeCV (%). He

Signal

Average

(CPS). Ar

SD (CPS).

ArCV (%). Ar

Cu 8.1E5 0.3E5 3.7 4.6E5 0.3E5 6.5

Zn 4.6E5 0.2E5 4.3 2.0E5 0.1E5 5.0

Ni 4.2E6 0.1E6 2.4 2.4E6 0.2E6 8.3

Mo 6.1E5 0.2E5 3.3 3.5E5 0.3E5 8.6

Cu/Zn 1.754 0.058 3.3 2.15 0.12 5.6

Ni/Mo 6.990 0.061 0.9 6.99 0.22 3.1

Table 2:Media, standarddeviation (SD), coefficientofvariation (CV) and samplesize (n) for Cu, Zn, Ni and Mo

signals and Cu/Zn and Ni/Mo ratios using He or Ar as ablation gas

The effect of two cyclone chambers with different sizes (~ 4 cm and ~6 cm internal diameter) was

evaluated. Figure 5 shows that per run, the signal is more stable when a cyclone chamber is used, because it

seems to dampen the effect produced in the ablation cell. Among ~ 4 cm and ~ 6 cm chambers no great

differences were obtained. Therefore, ~ 4 cm chamber was taken as condition routine to make smaller the

volume of the transport region.


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Figure 5:63Cu signal versus time with two cyclone chambers and without them. Ablation time was 50 s with

10 s of delay with 10 s delay relative to the acquisition start

In most of the works reported on solid samples, it is preferred ablate with a scanning speed different

to zero, because at the same point, it begins to form an increasingly deep crater with each impact of the laser.

The efficiency of energy transfer from the laser to the sample is continuously decreased and fractionation is

observed. With a scanning speed different to zero, ablation is carried out over a greater area, which ensures a

stoichiometric sampling.

Figure 6:63Cu signal versus time for the four scanning speeds indicated. Four replicates are shown. Ablation

time was 50 s with 10 s of delay relative to the acquisition start


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Four linear scanning speeds 0, 20, 40 and 60 m s-1 were tested in this work. Fig. 6 shows the

ablation profiles of the four scanning speeds with four repetitions each one. At 0 m s -1 reproducible profiles

with a signal drop from the beginning to the end of the run between 10 and 15% are observed. However, Fig.

7 shows that there is no fractionation of 63Cu/66Zn ratio at 0 m s-1. Profiles at speeds 20 and 40 m s -1are

little less stable and reproducible than at 0 m s-1

, but without signal drop. The profile at 60 m s-1

is morerandom probably for the larger sample area covering, which is not on the same horizontal axis (the crude oil

sample on holder may be concave or convex and it is not easy to reproduce a flat surface). Besides from the

beginning to the end of the run, its vertical position has changed, varying the level of defocus and so the

sensitivity. Even though, the 63Cu/66Zn ratio remained stable through time at four scanning speeds studied

(see Fig. 7). Finally, it canbe established thatthe most appropriate conditions fortheanalysisare

atsmallscanning speeds~ 20m s-1.

Figure 7:63Cu/66Zn ratio versus time at four scanning speeds. Signals were recorded in the same experiment

of Fig. 6

The fractionation effect versus fluence was evaluated with 15 elements by varying of defucus level

which allows to work in a greater range instead of varying the energy to a fixed diameter. As can be seen in

Fig. 8, the greater signal was obtained at higher positive defocus levels. Although fluence becomes smaller, it

is still sufficient to ablate a larger area the crude oil sample, which explains the results.


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Figure 8:60Ni, 63Cu, 66Zn and 95Mo signals versus defocus level. Each point represents the average of three 30

s runs of signal acquisition

Figure 9 shows the relationship between the elements studied and 66Zn (Zn was chosen for its high

ionization potential and high volatility) versus fluence. In general, fractionation was observed, i.e. M/66Zn

ratio increases with increasing fluence, except for Cd that appears to be stable, since it has very similar

characteristics to Zn. Zn and Cd that present this fractionation with respect to the other elements, have high

ionization potential and high volatility.

The observed behavior is hard to explain because ablation mechanisms are not yet fully known at the

date [15]. However, trends indicate thatwhentheablationis carried outbythermal processes, more

volatileelementsarepreferentiallyvolatilized [6]. Recent results[13] suggest thatelementswithlow ionization

potentialare preferentiallyableatedand coulomb interactions are responsible of that [11].

The results shown in Fig. 9 indicate that at lower fluences, Zn signal is favored compared to the other

elements (except for Cd), contrary effect to that observed in previous studies on glass samples [12,13]. The

results can not be explained by volatility if it is considered that there are greater thermal processes at high

fluencesor at leastgreatersurface damageby plasma-sampleinteraction (see Fig. 4a (less fluence) and 4c

(greater fluence, greatersurface damage)). As the Zn is more volatile, it should be favored at high fluence and

that does not happen. Similarly, the Mo that is an element with high melting and boilingpoint, presents

nofractionation, i.e. volatilitydoes not appear tobe responsible. The Al, that has relatively low ionization

potential, did not present fractionation. In conclusion, Zn and Cd were the only elements that showed

fractionation as function of fluence, but without sufficient evidence to explain it by volatility or ionization

potentials.


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Figure 9:Normalized M (isotopes indicated) / 66Zn ratios versus fluence (J cm-2). Each point represents three

2 min 30 s runs average of acquisition signal at a scanning speed of 20 m

It can be thought that the results of Fig. 9 may be influenced by mass load effect, because there is

greater quantity of particles at low fluences (greater defocus and larger diameter) than at high fluences (see

Fig. 8).

The ICP may be affected, decreasing its ionization temperature as a result of a large mass load, which

can cause different ionization profiles for elements with different ionization potentials.

To evaluate this effect with crude oil sample, the analysis with different crater diameters while

remained fixed the fluence was performed. Fig. 10 shows 63Cu/66Zn,95Mo/112Cd, 27Al/24Mg and 60Ni/63Cu

ratios which differ inionizationpotential (expressed as PIdenominator - PInumerator)1.68; 1.89; 1.66 y 0.09

eVrespectively. It was observed that ratios tend to increase when mass load increases. This effect was more

marked for the 95Mo/112Cd ratio, which has the largest difference in ionization potential. On the contrary

60Ni/63Cu ratio shows no significant change, since their PI not differ greatly.


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Figure 10:Normalized 63Cu/66Zn, 95Mo/112Cd, 27Al/24Mg and 60Ni/63Cu ratios versus crater surface. Each

point represents three 2 min 30 s runs average of acquisition signal at a scanning speed of 20 m

At higher mass load, the ICP diminishes its ionization temperature and those elements with high

ionization potential are more affected which explains the results of Fig. 10.

If mass load had an effect on the results of Fig. 9, they hada decreasing trend (because the greater

mass load is found in the low fluenceanalyse), confirming that the results are effects mainly produced by the

ablation processes at different fluences and not by the processes occurring in the ICP.

CONCLUSIONS

Fractionation was observed as function of fluence, i.e. Zn and Cd signals were affected in different

manner, respect to the other 13 elements studied, when fluence is increased. Elements with low ionization

potential as Al or low volatility as Mo, showed no fractionation when fluence is varied. Therefore, there was

insufficient evidenceto explainthe results in termsofthe volatility andtheionization potential. Craters pictures

are evidence that ablation mechanisms are changed at different fluence (see Fig. 4). They showed greater

surface damage at higher fluence. It can be probably due to plasma-sample interaction. Also, differences

among He and Ar ablation gases were observed. The best results, in terms of sensitivity and precision were

obtained when He was used as ablation gas. Fractionation was also observed in the ICP. At higher mass load

were disadvantaged those elements with high ionization potential due to the decreased of plasma ionization

temperature. Conditions of increased sensitivity to the extra heavy crude oil were determined in the LA-ICP-

MS system. It was found that they correspond mainly to a larger area of ablation (larger diameter and higher

defocus level), highlaser energy and high pulse frequency. Furthermore,an improvement inthe stability of

thesignal was observedwhen acyclonicchamberwas used inthe transport system, this may promote the use of


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various devices in the transport system to improve analysis precision of related samples with high organic

matter content. Under performed conditions, %RSD from 3 to 5% in the net signal of Cu, Zn, Ni and Mo and

from 0.9 to 3.3% in the Ni/Mo and Cu/Zn ratios were found, respectively. The results of this study show the

possibility of performing elemental determinations in extra heavy crude oil by ns-LA-ICP-MS. However,

further experiments are needed for the purpose of increasing the sensitivity of the analytes, improveprecision, reduce mass load effects and make the method robust to small matrix changes.

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STUDY OF NANOSECOND ND: YAG 213 nm LASER ABLATION OF EXTRA HEAVY CRUDE OIL BY INDUCTIVELY COUPLED...

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